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Subject: Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and
 ioq0
To:     Sagi Grimberg <sagi@grimberg.me>, Christoph Hellwig <hch@lst.de>
Cc:     keith.busch@intel.com, axboe@fb.com, linux-kernel@vger.kernel.org,
        linux-nvme@lists.infradead.org
References: <1519832921-13915-1-git-send-email-jianchao.w.wang@oracle.com>
 <20180228164726.GB16536@lst.de>
 <ddfc2ab4-1a51-c488-25e8-bc434edf8aba@grimberg.me>
From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <66e4ad3e-4019-13ec-94c0-e168cc1d95b4@oracle.com>
Date:   Thu, 1 Mar 2018 18:05:53 +0800
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Hi sagi

Thanks for your kindly response.

On 03/01/2018 05:28 PM, Sagi Grimberg wrote:
> 
>> Note that we originally allocates irqs this way, and Keith changed
>> it a while ago for good reasons.  So I'd really like to see good
>> reasons for moving away from this, and some heuristics to figure
>> out which way to use.  E.g. if the device supports more irqs than
>> I/O queues your scheme might always be fine.
> 
> I still don't understand what this buys us in practice. Seems redundant
> to allocate another vector without any (even marginal) difference.
> 

When the adminq is free, ioq0 irq completion path has to invoke nvme_irq twice, one for itself,
one for adminq completion irq action.
We are trying to save every cpu cycle across the nvme host path, why we waste nvme_irq cycles here.
If we have enough vectors, we could allocate another irq vector for adminq to avoid this.

Sincerely
Jianchao